1. Field of the Invention
This invention relates generally to mine roof bolt assemblies and, more particularly, to a mine roof bolt assembly having a plastically deformable Belleville-type washer.
2. Description of the Prior Art
It is a common practice in the mining industry to support mine roofs or ceilings with reinforcing plates or beams. Such reinforcing plates and beams are normally secured to the natural roof of the mine by means of roof bolt assemblies or the like. For proper safety, it is important that each of the roof bolt assemblies supporting the roof carries its proper load.
Typical mine roof bolt assemblies include a roof bolt, a bearing plate and a nut or washer for holding the bearing plate against the mine roof when the bolt is installed in a bolt hole drilled into the mine roof. Many of these mine roof bolt assemblies use threaded or non-threaded roof bolts having a collar for supporting the bearing plate against the mine roof. These bearing plates require both strength and reasonable weight for handling and are made of steel or other strong metal material. The plates may be embossed with a donut or annular shape on one side of the plate to increase strength. The bearing plate also includes a central aperture through which the roof bolt extends to secure the bearing plate to the mine roof.
In some roof bolt assemblies, anti-friction and/or standard flat washers are placed between the bearing plate and the head of the bolt. The purpose of the friction washer is to relieve friction between the plate and the nut or bolt head to thereby permit application of a more constant torque on the roof bolt during installation. These washers also help prevent the roof bolt head from being pushed through the aperture of the bearing plate.
U.S. Pat. No. 4,523,881 to Ringe discloses a typical embossed bearing plate held in place by a bolt and flat washer combination. U.S. Pat. No. 4,708,559 to Locotos discloses a roof bolt assembly using a rigid, non-deformable washer which is complementary to the embossment on the bearing plate.
Such previously known roof bolt assemblies provided with annularly embossed bearing plates and non-deformable washers are generally adequate to support normal loads of about 5,000 to 20,000 pounds but are totally inadequate for supporting heavy loads in excess of 30,000 pounds. In order to achieve higher load strengths with these prior art bolt assemblies, the thickness of the bearing plate is increased and the embossment may be removed so that the plate can be flattened for easier manufacture. However, by making the bearing plate thicker and flatter, the strength advantages obtained from the embossing are lost. In addition, substantially more material is required to produce the thicker, flat bearing plate than is required for a thinner, embossed plate.
Mine roof bolt assemblies incorporating a Belleville washer which acts as a tension indicator are also known. Belleville washers are elastic, frustoconical washers which have spring-like characteristics when compressed under a load. When the load is removed, the Belleville washer returns to its initial configuration. U.S. Pat. No. 2,850,937 to Ralston discloses a snap type bolt tension indicator in which an elastically deformable Belleville washer is placed between the reinforcing plate and the bolt head with the convex side of the Belleville washer facing the bolt head and the concave side of the Belleville washer facing the reinforcing plate. As the bolt is tightened, the Belleville washer reversibly deforms to lie against an annular ridge on the reinforcing plate. As the load on the bolt assembly varies, the Belleville washer flexes up and down to give a visual indication of the load on the bolt assembly. Loads of up to about 5,000 load pounds are reported in the Ralston patent.
It is an object of the invention to provide a mine roof bolt assembly having a Belleville-type washer which is able to substantially increase the load bearing capacity of an embossed bearing plate without significantly increasing the thickness and weight of the plate or unduly flattening the plate. The resultant bolt assembly thus substantially reduces the manufacturing costs for production of the bolt assembly while significantly improving the load bearing capacity of the bearing plate.